1. Field of the Invention
This invention relates to a composite article comprising randomly or regularly arrayed oriented microstructures partially encapsulated within a layer, in particular to the method of making the same and to the use of the composite article as an electrically conducting polymer, thin film resonant circuit, antenna, microelectrode or resistive heater, and as a multimode sensor to detect the presence of vapors, gases, or liquid analytes.
2. Background of the Invention
Composite articles containing or exhibiting a layered structure have been prepared by many different types of chemical and physical deposition processes.
For example, U.S. Pat. No. 4,812,352 discloses an article comprising a substrate having a microlayer (microstructured-layer) that comprises uniformly oriented, crystalline, solid, organic microstructures, several tens of nanometers in cross-section and a method of making the same. Further, '352 teaches optionally conformal coating the microlayer and encapsulating the conformal-coated microlayer.
Dirks et al. in "Columnar Microstructures in Vapor-Deposited Thin Films," Thin Solid Films, vol 47, (1977), pgs 219-33 review several methods known in the art that can yield columnar microstructures, however, as Dirks et al. point out the structures are not a desirable or a sought-after outcome of vapor-deposition.
U.S. Pat. No. 3,969,545 describes a vacuum deposition technique that can produce organic or inorganic microstructures.
Floro et al. in "Ion-Bombardment-Induced Whisker Formation of Graphite," J. Vac. Sci Technol A, vol. 1, no. 3, July/September (1983) pgs 1398-1402 describe graphite whisker-like structures produced by an ion-bombardment process.
Flexible conducting media known in the art, typically having a layered structure, exist in a variety of distinct formats. For example, U.S. Pat. No. 4,674,320 discloses a conducting powder-like material, such as carbon, dispersed throughout a polymeric binder at concentrations sufficient to enable conduction by charge transfer from particle to particle. Such an arrangement results in an isotropicly conducting sheet, that is, resistivity perpendicular to the plane of the sheet is the same as the in-plane resistivity.
Bartlett et al., Sensors and Actuators, vol. 20, pg 287, 1989, disclose a conductive polymer film made by electrochemical polymerization. Resistivities of these polymer films are three-dimensionally isotropic and tend to be relatively high.
Other examples known in the art teach an article comprising a conducting layer applied to a flexible polymer sheet by vacuum coating processes, electrochemical or electroless plating processes, printing, particle embedding and the like. However, in these cases, the conductive coating, for example, a solid metallic layer, will have a low resistivity and is not easily controllable. Additionally, since the conductive layer is on the surface of a polymer substrate, adhesion of the conductive layer to the polymer substrate is often a problem. The adhesion problem is particularly apparent when the conducting layer is carrying current. If a very thin or discontinuous conductive layer is applied to the polymer substrate to increase the surface resistivity, the power carrying capability of the conductive layer tends to be compromised and the problem of adhesion tends to be exacerbated.
Electrical properties are useful as sensors, however, most prior art gas and vapor sensors are based on many of the prior art layered structures. The sensor media can be thin or thick film devices utilizing either surface acoustic wave (SAW) technology or chemiresistors incorporating solid electrolytes, polymers with bulk gas sensitivity, metal or semiconductor (inorganic or organic) thin films, or homogeneous dispersions of conducting particles in insulating matrices.
Generally, sensors based on SAW technology are costly to manufacture and tend to be used only for reversible sensing. They are generally not used for nonreversible sensors, such as dosimetry monitoring, see Snow et al., "Synthesis and Evaluation of Hexafluorodimethyl carbinol Functionalized Polymers as SAW Microsensor Coatings," Polymer Reprints, 30(2), 213 (1989); Katritzky et al., "The Development of New Microsensor Coatings and a Short Survey of Microsensor Technology," Analytical Chemistry 21(2), 83 (1989).
On the other hand, chemiresistor based sensors tend to be reversible or nonreversible depending on the chemical and physical composition of the sensing medium, see Katritzky et al , "New Sensor Coatings for the Detection of Atmospheric Contaminants and Water," Review of Heteroatom Chemistry, 3, 160 (1990). Generally, the prior art sensing media exhibit isotropic or homogeneous gas sensing properties. Media having an isotropic sensing property display the same resistivity in all directions of the media. Such media are typically capable of only a single mode of detection. In contrast, media having an anisotropic impedance sensing property display different in-plane and out-of-plane gas sensing impedances. Thus, anisotropic media permit multi-mode operation.
Generally, conduction through chemiresistor devices occurs between conducting particles dispersed throughout the media. For example, U.S Pat. No. 4,674,320 teaches a chemiresistive gas sensor comprising a layer of organic semiconductor disposed between two electrodes, wherein dispersed within the layer of organic semiconductor is a high conductivity material in the form of very small particles, or islands. Adsorption of a gaseous contaminant onto the layer of organic semiconductor modulates the tunneling current.
U.S. Pat. No. 4,631,952 discloses an apparatus and a method for sensing organic liquids, vapors, and gases that includes a resistivity sensor means comprising an admixture of conductive particles and a material capable of swelling in the presence of the liquid, gas, or vapor contaminant.
Ruschau et al., "0-3 Ceramic/Polymer Composite Chemical Sensors," Sensors and Actuators, vol 20, pgs. 269-75, (1989) discloses a composite article consisting of carbon black and vanadium oxide conductive fillers in polyethylene, a polyurethane, and polyvinyl alcohol for use as chemical sensors. The polymer matrices swell reversibly in the presence of liquid and gaseous solvents, disrupting the conductive pathway and proportionally increasing the resistance.
U.S. Pat. No. 4,224,595 discloses an adsorbing type sensor having electrically conductive particles embedded in a surface, forming an electrically conductive path through the sensor.
U.S. Pat. No. 4,313,338 discloses a gas sensing device comprising a gas sensing element comprising a gas-sensitive resistive film formed of an aggregate of ultrafine particles of a suitable material deposited on the surface of a substrate of an electrical insulator formed with electrodes.
U.S. Pat. No. 3,820,958 discloses an apparatus and a method for determining the presence of hydrogen sulfide in a gas mixture. Silver is deposited on a thin dielectric film. Electrical resistance across the film before and after exposure of the film to hydrogen sulfide containing gas mixture is utilized to determine the amount of hydrogen sulfide present.
U.S. Pat. No. 4,906,440 discloses a sensor for a gas detector Comprising a metallic/metallic oxide gas sensitive discontinuous film. The gas changes the conductivity of the film and causes the RC network to react.
U.S. Pat. No. 3,045,198 discloses a detection device comprising an electrical element sensitive to exposure to liquids, vapors or gases. The detection element includes a broad and long base having an electrically non-conductive, relatively resilient surface on which is anchored a stratum of exposed electrically conductive discrete adsorbent particles.
Sadaoka et al., Effects of Morphology on NO.sub.2 "Detection in Air at Room Temperature with Phthalocyanine Thin Films," J of Mat'l Sci 25, 5257 (1990) disclose that crystal size in films is affected by the nature of the substrate, ambient atmosphere, and annealing time. The variations of the crystals can effect the detection of NO.sub.2 in air.